Conventional golf balls can be divided into two general classes: solid and wound. Solid golf balls include one-piece, two-piece (i.e., solid core and a cover), and multi-layer (i.e., solid core of one or more layers and/or a cover of one or more layers) golf balls. Wound golf balls typically include a solid, hollow, or fluid-filled center, surrounded by a tensioned elastomeric material, and a cover.
Golf ball core and cover layers are typically constructed with polymer compositions including, for example, polybutadiene rubber, polyurethanes, polyamides, ionomers, and blends thereof. Ionomers, particularly ethylene-based ionomers, are a preferred group of polymers for golf ball layers because of their toughness, durability, and wide range of hardness values.
Ionomers are stiffened by increasing the amount of neutralization. Neutralization to 70% and higher, including 100%, has been disclosed. However, in the absence of flow additives, at neutralization levels above about 60%, the melt flow of the ionomer is decreased to an extent such that processability decreases or disappears altogether. For trivalent cations, the percent neutralization at which the polymer becomes unprocessable, in the absence of flow additives, can be significantly lower.
Blending ionomers with fatty acids is a known method for improving the processability of highly neutralized polymers. For example, U.S. Pat. No. 6,653,382 discloses melt-processable, highly-neutralized ethylene acid copolymers and a process for making them by incorporating an aliphatic, mono-functional organic acid in the acid copolymer and then neutralizing greater than 90% of all of the acid groups present.
Another known method for improving the processability of highly neutralized polymers is blending ionomers with oxa acids. U.S. Patent Application Publication 2003/0225197 discloses a process for forming a golf ball having at least one layer, wherein the layer is formed of a polymer blend including a highly neutralized polymer formed from an oxa acid, a thermoplastic resin, and an inorganic metal compound, an organic amine, or a combination thereof, wherein greater than about 70% of the acid groups in the polymer blend are neutralized.
Blends of polyamides and highly neutralized ionomers are generally known. For example, U.S. Pat. No. 5,866,658 discloses blends of highly neutralized ionomers of copolymers of ethylene and high weight percentages of α,β-ethylenically unsaturated C3-C8 carboxylic acids dispersed in a continuous or co-continuous polyamide phase. Such blends are disclosed as being particularly useful in automotive applications, but their use in golf ball applications is not disclosed.
The use of polyamide-ionomer graft copolymers in golf ball compositions is known. U.S. Patent Application Publication No. 2002/0013413 and U.S. Pat. No. 6,579,938 disclose a golf ball with a cover or mantle, or both, formed from a composition comprising a polyamide-ionomer graft copolymer and a method of making the same. The ionomers are preferably neutralized from about 30% to about 65%. The references do not disclose neutralization of the ionomer above 90%.
A desire remains for novel compositions containing a highly neutralized acid polymer (“HNP”), which are useful in golf ball applications, and which are processable without the need for fatty acids or their salts. The present invention describes such compositions and their use in a variety of golf ball core and cover layers.